Alloy cast iron

ABSTRACT

A HEAT-TREATED ALLOY CAST IRON CONSISTING ESSENTIALLY OF 10 TO 15% OF A NICKEL EQUIVALENT COMPONENT CONSISTING OF A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, NICKEL EQUIVALENT METALS AND MIXTURES THEREOF, AT LEAST ABOUT 8% OF SAID ALLOY BEING NICKEL, SAID NICKEL EQUIVALENT METALS BEING PRESENT IN AN AMOUNT NOT OVER ABOUT 8% OF SAID ALLOY AND BEING SELECTED FROM THE GROUP CONSISTING OF COPPER AND MANGANESE, THE AMOUNT OF COPPER NOT EXCEEDING ABOUT 3% OF SAID ALLOY, THE AMOUNT OF MANGANESE, WHEN PRESENT AS AN INTENDED ALLOY ADDITION, NOT EXCEEDING ABOUT 2% OF SAID ALLOY, THE BALANCE OF THE ALLOY CONSISTING ESSENTIALLY OF IRON AND CARBON, AND ANY MANAGANESE PRESENT AS AN INCIDENTAL CONSTITUENT, UP TO 1% OF SAID ALLOY, NOT BEING INCLUDED IN SAID NICKEL EQUIVALENT METAL AMOUNT, THE PRESENCE OF CHROMIUM BEING LIMITED TO A RESIDUAL AMOUNT NOT EXCEEDING ABOUT 0.07% OF SAID ALLOY, ALL PERCENTAGES BEING BY WEIGHT, SAID ALLOY HAVING BEEN CAST INTO AN UNSTABLE AUSTENITIC STRUCTURE WHICH MAY CONTAIN MARTENSITE, THEN HEAT-TREATED TO TRANSFORM SAID   AUSTENITIC CAST IRON INTO A BAINO-MARTENSITIC STATE OF MAXIMUM HARDNESS, SAID HEAT-TREATMENT BEING CONTINUED FOR A SUFFICIENT LENGTH OF TIME TO CAUSE A DECREASE IN HARDNESS INTO THE RANGE BELOW SAID MAXIMUM, TO INCREASE MECHANICAL STRENGTH, THERMO-STABILITY AND TOUGHNESS TO THE ALLOY.

C H A RES R. vm on B: J0sep A P INVENTORQ y 1972 c. R. VAN-DER BEN ET L ALLOY CAST IRON Original Filed March 1, 1967 95 QR Q9 QQM oow Q9 Q3 dam United States Patent 015cc 3,674,576 Patented July 4, 1972 US. Cl. 14835 11 Claims ABSTRACT OF THE DISCLOSURE A heat-treated alloy cast iron consisting essentially of to of a nickel equivalent component consisting of a metal selected from the group consisting of nickel, nickel equivalent metals and mixtures thereof, at least about 8% of said alloy being nickel, said nickel equivalent metals being present in an amount not over about 8% of said alloy and being selected from the group consisting of copper and manganese, the amount of copper not exceeding about 3% of said alloy, the amount of manganese, when present as an intended alloy addition, not exceeding about 2% of said alloy, the balance of the alloy consisting essentially of iron and carbon, and any manganese present as an incidental constituent, up to 1% of said alloy, not being included in said nickel equivalent metal amount, the presence of chromium being limited to a residual amount not exceeding about 0.07% of said alloy, all percentages being by weight, said alloy having been cast into an unstable austenitic structure which may contain martensite, then heat-treated to transform said austenitic cast iron into a baino-martensitic state of maximum hardness, said heat-treatment being continued for a sufiicient length of time to cause a decrease in hardness into the range below said maximum, to increase mechanical strength, thermo-stability and toughness to the alloy.

This application is a streamline continuation of Ser. No. 619,743, filed Mar. 1, 1967, which in turn is a continuation-in-part of Ser. No. 582,161, filed Sept. 26, 1966, which in turn is a streamline continuation of Ser. No. 311,224, filed Sept. 24, 1963, all of which are now abandoned.

This invention concerns an alloy cast iron, more particularly it concerns the heat treatment of a particular alloy to obtain highly desirable properties.

With the continued development of internal combustion engines and similar machinery to obtain increased efiiciencies and powers a need arises for new materials for their construction capable of withstanding conditions not previously encountered.

The invention is the result of a metallurgical research program carried out with a view to producing a machinable alloy cast iron having axceptionaly high strength and shock resistance and having structural stability over prolonged periods at temperatures of up to 500 C., and suitable for the construction of components of internal combustion engines or similar machinery, subject to high mechanical and thermal stresses and shock.

Normal plain (unalloyed) pearlitic grades of engineering irons are prone to structural breakdown on prolonged heating at temperatures in the range of 400450 C. and are subject to sharply falling mechanical properties at temperatures above about 450 C. on short time heating and have poor creep resistance at such temperatures.

The normal range of alloyed pearlitic irons have rather better properties, depending upon the amounts and combination of alloying elements employed and the general composition balance but do not give the marked stability sought, this also applies to S.G. Iron. Many special alloy irons have been developed for specific applications (e.g. resistance to heat, corrosion, scale, wear, etc. and other properties such as magnetic, electrical, etc.) but these do not give the combination of properties desired.

According to the present invention an alloy cast iron consists essentially of a component equivalent to from 10 to 15% of nickel and consisting of a metal taken from the class consisting of nickel, nickel equivalent metals and mixtures thereof, at least about 8% of such metal being nickel, the nickel equivalent metals being present in an amount not over about 8% and being taken from the class consisting of copper and manganese, the amount of copper (if present) not exceeding about 3%, the amount of maganese (if present as an intended alloy addition) not exceeding about 2% the balance consisting essentially of iron and carbon, any manganese present as in incidental constituent (up to 1%) not being included in said nickel equivalent, the presence of chromium being limited to a residual amount not exceeding about 0.07%, all percentages being by weight, characterised in that the alloy is heat treated from an as cast unstable austenite (which, subject to casting sections may contain some martensite) to form a product having a structure lying in the zone hereinafter defined.

By one percent nickel equivalent we mean both here and throughout the specification and claims that percentage of one of the alloying elements other than nickel which is equivalent in its effect to 1% of nickel in producing an austenitic as cast structure. Thus, for example 1% of manganese and 10% of nickel together have a nickel equivalent percentage of 13, since manganese is approximately three times as effective as nickel in poducing an austenitic as cast structure. Again 1% of copper and 10% of nickel together have a nickel equivalent percentage of 11 since copper is approximately equivalent to nickel in producing an austenitic as cast structure, providing that the nickel content is not less than approximately 3 times the copper content.

The invention will be further apparent from the following description with reference to the single figure of the accompanying drawing, which shows a graphical representation of the effect of heat treatment on alloys having a composition in accordance with the invention.

As we have seen an alloy embodying the invention is formed by the casting of a fairly critical composition followed by a fairly critical heat treatment.

An analysis of four typical alloys within the field of our invention revealed the following results.

Percent Alloy I II III IV 3. 21 3. 66 3. 16 2. 40 1. 43 1. 62 1. 50= 2. 20 Phosphorus 0. 04 0. 03 0. 06 0. 06 Sulphur. 0. 01 0. O1 0. 01 0. 02 Manganese 1. 00 0. 98 2. 76 0. 12. 50 10. 70 7. 50 10. 70 Nil Nil 1. 95 Nil 0. 34 Nil 0. 32 Nil Nil 0. 37 Nil 1. 0

Alloys I, II and III, were speriodal graphite irons and alloy IV was a flake graphite iron.

It will be noted that the four alloys each have a combined nickel and nickel equivalent percentage lying in the range of from to Such combined percentages being 12.5, 10.7, 14.73 and 10.7 respectively, bearing in mind that the first 1% of manganese is ignored for the purpose of calculating the nickel equivalent percentage, since up to 1% of manganese is normally present in an alloy cast iron as an incidental constituent. In no case does the percentage of copper exceed 3 whilst the percentage of manganese (excluding the first 1%) does not exceed 2, and a programme of tests has shown that these limits must be approximately adhered to in order to achieve the desired product. Chromium if present should only be in residual amounts not exceeding about 0.07%.

All the above examples of alloy were found after casting to have an unstable austenitic structure though traces of martensite were present depending upon the casting sections.

It will be seen that the graph shows a plot of hardness against heat treatment time. Curves A and B respectively show the eifect of isothermal heat treatment on the material, whence it will be seen that with the passage of time the structure transforms from an austenitic through a bainomartensitic state of maximum hardness. We have discovered that the continuation of the heat treatment after the fully hardened condition is reached eventually produces a further change of structure marked by a crop in hardness, and we have discovered that this drop in hardness is accompanied by exceptional increases in toughness and thermal structural stability at temperatures up to 500 C. Whilst maintaining exceptionally high strength.

For the purpose of defining our invention We designate any alloy having a composition of the kind in question which has been heat treated at least for a sufficient length of time for the hardness to begin the drop as described above as a material which has been heat treated to form a product having a structure lying in the zone herein defined.

It will be clear from the graph that the heat treatment times required when using an isothermal heat treatment to obtain a product having a structure in the zone defined are so large as to be impracticable, but we have found by utilising an athermal heat treatment the heat treatment time required can be significantly reduced as can be seen from curve C on the graph.

In order to illustrate suitable heat treatment for processing alloys lying within the field of this invention we shall now describe the heat treatments applied to the four typical alloys mentioned hereinabove.

Heat Treatment. Iron No. I

Heated to 900 C. and held for 2 hours Cooled to 700 C. and held for 24- hours 130 h Cooled to 600 C. and held for 48 hours approx Cooled to 500 C. and held for 48 hours Furnace cooled to room temperature.

Iron No.s II and IV Heated to 900 C. and held for 2 hours Cooled to 800 C. and held for 24 hours Cooled to 700 C. and held for 24 hours 150 hrs. approx.

Cooled to 600 C. and held for 100 hours Furnace cooled to room temperature.

Iron No. III

Furnace cooled to room temperature.

It will be noted that the heat treatments applied to Irons I, II and IV were arrest and soak methods whilst that applied to Iron III was a thermal cycling method.

We carried out a number of tests to determine the properties of the heat treated alloys and the results of such tests are summarised in the following table. The test results for a normalised pearlitic S.G. iron are included on the table by way of comparison.

Mechanical Tests Comparison with normalised earl itic G. I II III iron.

LP. at-

20 0., tons/sqJin. 39. 6 60.0 37. 6 37. 5 500 0., tons/sqJinh..." 31. 8 32.6 Max. stress at 20 0., tons/sq/in 60. 0 85. 0 62.0 35.0 57. 5 600 0., tons/sqJ in 41.0 49.0 Elongation, percent at 20C 10.0 5.0 5.0 00 C.-. 12. 0 R. I. at 20 0-- ,300 15, 000 Brinell hardness 2 5 400 350 2 0 320 1 Limit of proportionality. 1 Repeated impact (Stanton test) blows to fractur .Blow energy 0.20 t.lbs.

In order to demonstrate the thermal stability of the heat treated alloys we give below a table of tests carried out on Iron I after it had been held at 550 C. for 1000 hours. Again we include the results obtained for a normal pearlitic S.G. iron for comparison purposes.

Tests After Heating for 1,000 Hours at 550 0.

Longer heating results in further deterioration of the pearlitic S.G. iron whilst no change is produced in our special iron.

It will be appreciated that there are many difierent compositions within the field of the invention. Thus for example molybdenum may be included up to 1% by weight in the alloy for the purpose of increasing hot strength and creep resistance if required. Additions of niobium of up to 1% by weight may be made to give rise to finely dispersed niobium carbide particles which appear to act in a similar manner to that of a precipitation hardening and strengthening phase, the form distribution and efiect being completely difiereut from the carbides produced by chromium, which by their massive form considerably increase hardness but also increase brittleness when present in quantity in a grey iron.

Alloys may be cast so that the carbon not in solution is in either spheriodal or flake condition.

What is claimed is:

1. A heat-treated alloy cast iron consisting essentially of 10 to 15% of a nickel equivalent component consisting of a metal selected from the group consisting of nickel, nickel equivalent metals and mixtures thereof, at least about 8% of said alloy being nickel, said nickel equivalent metals being present in an amount not over about 8% of said alloy and being selected from the group consisting of copper and manganese, the amount of copper not exceeding about 3% of said alloy, the amount of manganese, when present as an intended alloy addition, not exceeding about 2% of said alloy, the balance of the alloy consisting essentially of iron and carbon, and any manganese present as an incidental constituent, up to 1% of said alloy, not being included in said nickel equivalent metal amount, the presence of chromium being limited to a residual amount not exceeding about 0.07% of said alloy, all percentages being by weight, said alloy having been cast into an unstable austenitic structure which may contain martensite, then heat-treated to transform said austenitic cast iron into a baino-martensitic state of maximum hardness, said heat-treatment being continued for a sufficient length of time to cause a decrease in hardness into the range below said maximum.

2. A heat treated alloy cast iron according to claim 1 wherein the heat treatment comprises the steps of heating the alloy composition to about 900 C. for homogenization thereof and subsequently cooling the alloy to a temperature of about 500 C. over a period of at least about 100 hours.

3. A heat treated alloy cast iron according to claim 2 wherein the cooling of the alloy composition from about 900 C. is efiected in a stepwise or arrest and soak manner.

4. A heat treated alloy cast iron according to claim 1 wherein the heat treatment comprises the steps of heating the alloy composition to about 900 C. for homogenization thereof, cooling the alloy composition at a temperature in the range of 500-600 C, and alternately reheating and recooling the alloy composition, the cooling in each case being to a temperature in the range of 500-600 C. and the reheat temperature at each successive cycle being less than at the previous cycle.

5. A heat treated alloy cast iron according to claim 1 wherein the heat treatment comprises the steps of heating the alloy to a temperature of about 900 C., cooling the alloy composition to a temperature of about 500 C. and maintaining the alloy composition at a temperature of about 500 C. for at least about 300 hours.

6. A heat treated alloy cast iron according to claim 1 wherein the nickel content lies in the range of from about to 13%.

7. A heat treated alloy cast iron according to claim 1 wherein the carbon not in solution is present in spheroidal condition.

8. A heat treated alloy cast iron according to claim 1 wherein the carbon not in solution is present in flake condition.

9. A heat treated alloy cast iron according to claim 1 including niobium as an additive in an amount of up to 1% by weight.

10. A heat treated alloy cast iron according to claim 1 including molybdenum as an additive in an amount of up to 1% by weight.

11. The heat-treated alloy according to claim 1 exhibiting increased mechanical strength, increased toughness and increased thermo-stability as a result of said heat treatment.

References Cited UNITED STATES PATENTS 1,988,910 1/1935 Mercia -1 2,485,760 10/1949 Millis 75--l23 2,842,437 7/1958 Guenzi 75125 3,253,949 5/1966 Larin 75-123 J 3,318,423 5/1967 Dunki 75-123 X OTHER REFERENCES Angus, Harold T., Physical and Engineering Properties of Cast Iron, England, The Britich Cast Iron Research Assn., 1960, pp. 208-211; 367-375.

L. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner US. Cl. X.R. 

